Method and device for distance measurement

ABSTRACT

The present invention relates to a device ( 10 ) for distance measurement, with at least one transmitting branch ( 14 ) with a transmission source ( 22, 24 ) for a measurement signal for emitting a modulated measuring beam ( 16, 26, 36 ) in the direction of a target object ( 20 ), and with a receive branch ( 18 ) for the measurement radiation ( 17, 44 ) returning from the target object ( 30 ), and with a control and evaluation unit ( 28, 58 ) for determining the distance of the device ( 10 ) to the target object ( 20 ) from the measurement radiation returning from the target object ( 20 ).  
     It is proposed according to the invention that the device ( 10 ) include means that enable measurement of distances with predetermined measurement uncertainties. The present invention further relates to a method for distance measurement, with which a measurement of distances with predetermined measurement uncertainties is possible. To ensure a distance measurement in a certain, predetermined measurement time, the value on which a distance measurement is based can be adjusted to the measurement uncertainty, and can be increased incrementally in particular.

The present invention is directed to a device for distance measurementaccording to the definition of the species in claim 1 and/or a methodfor distance measurement according to the definition of the species inclaim 15.

BACKGROUND INFORMATION

Distance measurement devices and, in particular, optical distancemeasurement devices as such have been known for some time. These devicesemit a modulated measuring beam, e.g., a light or laser beam, which isdirected to a desired target object, the distance of which to the deviceis to be determined. A portion of the returning measurement signal thatis reflected or scattered by the sighted target object is detected againby the device and used to determine the sought-after distance.

A distinction is made hereby between “phase measurement methods” andpure “transit time methods” for determining the sought-after distance tothe target object. With the transmit time method, a light pulse havingthe shortest possible pulse duration is emitted by the measurementdevice, then its transit time to the target object and back to themeasurement device is determined, for instance. Based on this, thedistance of the measurement device to the target object can becalculated, with reference to the known value for the speed of light.

With the phase measurement method, in contrast, the variation of thephase of the measurement signal with the path that was covered is usedto determine the distance between the measurement device and the targetobject. Based on the magnitude of the phase displacement of thereturning light in comparison to the emitted light, the path covered bythe light and, therefore, the distance of the measurement device to thetarget object can be determined.

The field of application of distance measurement devices of this typegenerally includes distances in the range of a few millimeters up tomany hundred meters. Depending on the paths to be measured, theenvironmental conditions and the reflectance of the selected targetobject, different requirements on the performance of a measurementdevice of this type result. Measurement devices of this type are nowavailable commercially in compact designs; they enable simple, handheldoperation for the user.

Laser distance measurement devices are known that have a definedmeasurement accuracy that is defined essentially by the measurementsystem on which the measurement device is based. This accuracy of thedistance measurement device is guaranteed for a specified measurementrange of the measurement device, by the manufacturer, for instance.

A circuit arrangement and a method for optical distance measurement isknown from DE 198 11 550 A1, for example, with which at least twodifferent, closely adjacent measurement frequencies are derived from anoscillator. To permit measurement over the greatest possible measurementrange and, simultaneously, to obtain the highest possible measurementaccuracy in the distance measurement, three different frequencies in therange from approximately 1 MHz to approximately 300 MHz are used in themethod according to DE 198 11 550 A1, and the sought-after path ismeasured with each of these frequencies.

An optical method for measuring distances according to the pulse transittime method is known from EP 0 885 3999 B1, with which a roughmeasurement procedure and a fine measurement procedure are carried out.Using a rough measurement procedure, a measurement time interval isdetermined that is greater than an estimated propagation time of thelight signal to and from the desired target object. An appropriatemeasurement time range is fixed in advance within this measurement timeinterval. A series of sub-measurements is then carried out during thefine measurement procedure, whereby, for each sub-measurement, ameasurement light signal is sent to the target object and the received,returned light pulse that is reflected by the target object is collectedonly within the appropriate measurement time range that was fixed duringthe rough measurement procedure. The exact distance of the measurementdevice to the target object is then determined by calculating the meanof the individual measurements in the fine measurement procedure.

The object of the present invention is to expand, using simple means,the distance range—that is, the distance range across which a distancemeasurement can be carried out with the device-that is usable with acompact, and, in particular, handheld measurement device for distancemeasurement.

This object is attained via a device for distance measurement accordingto the invention with the features of claim 1 and via a method fordistance measurement with the features of claim 14.

Advantages of the Invention

In contrast to devices of the related art, the device according to theinvention and/or the method according to the invention have theadvantage that they enable distances to be measured with differentmeasurement accuracies. If a measurement accuracy is guaranteed and,therefore fixed, over a certain range of measurement distances, thenthis is a limiting criterium—due, e.g., to the decrease in signalintensity with the distance—for the measurement distance still to bedetermined with the predefined measurement uncertainty. The measurementuncertainty of a measurement is essentially determined by thesignal-to-noise (S/N) ratio of the measurement signal. For smallreflected signals, which occur when measurement distances are great orwhen a measurement is taken against surfaces with low reflectance, thisresults in a limitation of the measuring range which can be measuredwith a specified measurement uncertainty. If the measurementuncertainties with which a corresponding distance measurement is carriedout are not fixed, but rather can be predetermined by the user or via anautomatic procedure in the device, then the measurement range—which isaccessible with a measurement device and/or, correspondingly, a methodof this type—over which the distance measurements are possible can bemarkedly expanded, even if a higher measurement uncertainty must betolerated.

For a large number of areas of application of a, e.g., handheld, compactdistance measurement device, the advantages resulting from the expansionof the measurement range outweigh the possible disadvantages of greatermeasurement uncertainty and/or reduced measurement accuracy.

Advantageous further developments of the device indicated in theindependent claims and/or the claimed method are possible due to themeasures listed in the dependent claims.

The measurement inaccuracy of the measurement device may be optimallyadapted to the particular measurement task in an advantageous manner. Inmany cases of the typical use of a compact distance measurement deviceof this type, high accuracy with a resolution in the range of a fewmillimeters is not required, for example. When measuring largerdistances, in particular, it is desirable to first obtain a firstmeasured value and starting point for the sought-after distance, sothat, in this case, a determination of the sought-after distance with anaccuracy of a few millimeters is not even required. Much too muchmeasurement expenditure would be required to carry out distancemeasurements over a distance of a hundred or more meters with the samelow level of measurement uncertainty as it would require to carry out ameasurement over a few meters.

With the device for optical distance measurement according to theinvention, it is advantageously possible to markedly expand the range ofdistances to be measured, in principle, with a device of this type.Instead of a fixed, predetermined measurement uncertainty of distancemeasurement and/or a corresponding resolution of the distance measured,a variable measurement uncertainty is made possible with the distancemeasurement carried out by the device. For example, the measurementdistance of a distance measurement device of this type may be expandedmarkedly when the requirements on the measurement uncertainty of thevalue to be determined are reduced for the range of greater measurementdistances, e.g., in the range of 50 to many hundred meters. Likewise,the measurement time needed to determine a measurement distance may bemarkedly reduced when the measurement uncertainty of the measurementsystem is raised accordingly for this purpose.

In advantageous fashion, a number of characteristic curves, e.g.,characteristic curves that specify the course of the measurementuncertainty over a measurement distance, can be stored in a storagemedium of the measurement device for this purpose.

Based on a target entered by the user, e.g., using a keypad on thedevice or via an automated system internal to the device, acharacteristic curve can then be selected that specifies—as a functionof the measurement distance—a measurement uncertainty on which thedistance measurement is to be based.

For example, in an advantageous embodiment of the measurement deviceaccording to the invention and/or the method for distance measurement onwhich it is based, a maximum measurement time for a measurement can thenbe predetermined, and the device switches automatically between theavailable characteristic curves for the measurement uncertainty toselect that characteristic curve—with consideration for thepredetermined measurement time—which ensures the lowest possiblemeasurement uncertainty.

In this manner, it is ensured that the minimum measurement uncertaintyof the device is utilized in the range of small measurement distancesand the measurement uncertainty does not gradually become greater untilthe distances are greater, so that an expanded measurement range is madeavailable to the measurement device without the measurement uncertaintybecoming too great in the range of small measurement distances.

In advantageous fashion, a value for the signal-to-noise ratio (S/N) ofthe returning amplitude signal to be detected can be specified to thecontrol and evaluation unit of the device. This signal-to-noise ratiothen essentially defines the accuracy with which a distance measurementis to be carried out.

In a likewise advantageous manner, the distance measurement deviceaccording to the invention is configurable such that the measurementtime, the measurement uncertainty of the measurement and the resolutionof the measured result are selectable individually or as a whole. A userof the measurement device according to the invention can enter a fixedmeasurement time or a desired accuracy for the distance measurementusing an operating field, for example. The electronics in themeasurement device then adjust, semi-automatically, the remainingmeasurement parameters via corresponding circuit means in such a mannerthat the desired measurement uncertainty and/or the desired measurementtime are made possible.

The measurement device according to the invention can be set in thismanner for a measurement uncertainty of 10⁻³ m, for example, for workingat close range up to approximately 10 m on highly reflective surfaces,whereby the measurement time would amount to one second at most, forexample, and the resolution of the measurement device should be 10⁻⁴ m.This setting may result in it being impossible to carry out ameasurement on dark surfaces, which is irrelevant for the user's desiredmeasurement situation anyway. The measurement device may also beconfigured optimally in a likewise advantageous manner for working atfar range, e.g., between 50 m and 100 m, by reducing the accuracy of themeasurements to 10⁻¹ and setting the resolution of the determinedmeasured value to 10⁻² m.

In an exemplary embodiment of the measurement device according to theinvention, a sensor is integrated that detects the light conditions inthe environment of the measurement site and, based on this, determines ameasure of the background signal that exists for a measurement. Thisbackground signal is incorporated in the signal-to-noise ratio thatexists for a measurement and therefore influences the possiblemeasurement uncertainty of a distance measurement. In an advantageousembodiment, this sensor function is performed by the detector element ofthe receive branch, so that the measurement signal and the backgroundsignal are both determined with only one detector.

An automatic switching over of the measurement uncertainty of the devicebased on the relative intensity of the ambient light can bepredetermined in the method according to the invention and integratedaccordingly in a measurement device that operates according to thismethod.

For example, the possible distance measurement range for a maximumpredetermined measurement time can be expanded by reducing therequirements on the signal-to-noise ratio across the distance. Whenworking outdoors in sunlight, i.e., with a strong background and/ornoise signal, in particular, this results in a marked increase in theusability of the measurement device according to the invention.

Advantageously, only one measurement parameter (measurement time,resolution of the distance, measurement uncertainty, . . . ) can befixed in the evaluation unit of the measurement device according to theinvention, for example, so that the other measurement parameters areadjusted semi-automatically by the control electronics of themeasurement device such that, given a fixed setting, e.g., themeasurement time, the sought-after distance is determined with thebest-possible accuracy, i.e., with minimal measurement uncertainty. Thisresults in a depiction of the measured value that is adjusted for theresolution used, however.

The device for optical distance measurement according to the inventionalso makes it possible for the device to configure itself, independentlyand fully automatically, such that all parameters are adjusted such thatan optimum setting of the measurement parameters is carried outdepending on the distance and environmental conditions.

In an exemplary embodiment according to the invention, the value of thesignal-to-noise ratio which determines the measurement accuracy isdetermined by a first “rough” measurement of the distance to the targetobject by the device itself, the rough measurement being carried outbefore the actual distance measurement. The subsequent, secondmeasurement for determining the distance between the measurement deviceand the target object is then carried out with an accuracy and,therefore, measurement time requirement, that is adjusted to the roughdistance range.

In an advantageous exemplary embodiment of the device according to theinvention, various measurement uncertainties are set up for thispurpose, which are allocated to individual distance intervals. Based onthe approximate distance determined in the rough measurement, ameasurement uncertainty for the actual distance measurementcorresponding to this distance is then selected by the device.

It is also possible with the method according to the invention that theuser himself specifies the resolution of the distance before ameasurement by entering “mm”, “cm”, or “m” via an operating field, forexample, and the measurement device selects an adjusted measurementuncertainty—with consideration for the measurement situation, that is,with consideration for the level of the background signal and thedesired measurement time, for example—that is, it determines thesignal-to-noise ratio up to which the measurement should be carried out.While the measurement is underway, the particular currentsignal-to-noise ratio is then determined by a control and evaluationunit of the measurement device, and a decision is made as to whether themeasurement must be carried out for a longer period of time.

It is particularly advantageous when a plurality of characteristiccurves is stored in the measurement device, which have a differentcourse of measurement uncertainty with the measurement distance, sothat, by selecting a characteristic curve of this nature, a measurementuncertainty that is still acceptable for a selected measurement range isselected.

This can also take place, for example, by a user roughly specifying anapproximate distance range, and the device then selecting acorresponding, optimized characteristic curve for the measurementuncertainty.

In advantageous fashion, in the measurement device according to theinvention, the setting for the accuracy of the length measurement onwhich a distance measurement is based is shown to the user via anoptical display. For example, via a display of “millimeters”,“centimeters”, or “meters”, the user can be informed immediately as towhich magnitude the measured result appearing in the display for hislength measurement can be specified in and be accurate.

In a further exemplary embodiment of the device according to theinvention, the display of the measured results of a distance measurementcan be depicted, for example, with the number of decimal placescorresponding to the accuracy of the distance measurement, in a displaydevice of the measurement device. The measurement accuracy, whichdecreases as the measurement distance increases, can be visualized forthe user of the measurement device in a simple but unequivocal manner byreducing the display resolution, for example.

The method, according to the invention, for distance measurement withphase displacement of amplitude-modulated light makes it possible in asimple and advantageous fashion to markedly expand the length range forthe distance measurement that is possible with a measurement device ofthis nature. As an alternative, the method according to the inventionmakes it possible to reduce the measurement time for a measurement givena typical, predetermined distance from a target object, for example. Themeasurement range that is accessible with the method for distancemeasurement according to the invention is no longer limited by ameasurement accuracy that is specified once and applies across theentire measurement range and for all applications of the device;instead, it can be markedly expanded in simple fashion by adapting themeasurement accuracy to the measurement task. The method according tothe invention permits the area of application of a measurement device ofthis type to be expanded markedly.

Further advantages of the device according to the invention and/or themethod according to the invention result from the drawings and theassociated description.

DRAWING

An exemplary embodiment of the device according to the invention and themethod for optical distance measurement according to the invention aredepicted in the drawing. The exemplary embodiment will be described ingreater detail in the subsequent description. The figures in thedrawing, their description, and the claims directed to the presentinvention contain numerous features in combination. One skilled in theart will also consider these features and/or the claims on which theyare based them individually and combine them to form further reasonablecombinations and claims.

FIG. 1 shows a device according to the general class for opticaldistance measurement in a schematic total overview,

FIG. 2 is a flow chart with the essential method steps on which themethod according to the invention is based,

FIG. 3 shows the schematic course of the measurement uncertainty of ameasurement device across the measurement distance, and a series ofcharacteristic curves—which can be entered in advance in the deviceaccording to the invention—of the measurement uncertainty as a functionof the measurement distance as examples, and

FIG. 4 shows the schematic course of the measurement time across themeasurement distance in the case of an essentially constant measurementuncertainty and in the case of a measurement in accordance with thepredetermined characteristic curves according to FIG. 3.

FIG. 1 shows, in schematic fashion, a distance measurement device 10according to the general class with the most important components fordescribing its basic configuration. Device 10 has a housing 12 in whicha transmission branch 14 for generating a measurement signal 16 and areceive branch 18 for detecting the measurement signal 17 returning froma target object 20 are located. Receive branch 18 forms a receivechannel for returning measurement signal 17.

Transmit branch 14 contains a light source 22, which is realized in theexemplary embodiment in FIG. 1 by a semiconductor laser diode 24. Theuse of other light sources and non-optical transmitters in the deviceaccording to the invention is also possible.

Laser diode 24 in the exemplary embodiment according to FIG. 1 emits alaser beam in the form of a light bundle 26 that is visible to the humaneye. Laser diode 24 is operated via a control device 28 which, usingcorresponding electronics, generates a modulation of the electricalinput signal 30 to diode 24. Control device 28 receives the necessaryfrequency signals to modulate a control and evaluation unit 58 of themeasurement device. In other exemplary embodiments, control device 28can also be an integral component of the control and evaluation unit 58.

Control and evaluation unit 58 includes a circuit arrangement 59 whichalso includes, among other things, at least one quartz oscillator forproviding the necessary frequency signals. The measurement signal ismodulated in known fashion with these signals of which typically aplurality having different frequencies is used during a distancemeasurement. The principle configuration of a circuit arrangement ofthis type is described in publication DE 198 11 550 A1, for example, andwill therefore not be explicitly repeated here.

Intensity-modulated light bundle 26 exiting from semiconductor diode 24passes through first optics 32, which results in an improvement of thebeam profile of the light bundle. Optics of this type can also be anintegral part of the laser diode itself. Laser beam bundle 26 thenpasses through a collimation lens 34, which generates a nearly parallellight beam bundle 36 that is emitted in the direction of target object20 to be measured. For this purpose, a device 38 for generating areference distance 40 internal to the device is located in transmitbranch 14 of the device according to FIG. 1, the reference distanceserving as internal calibration of the measurement device.

Measurement signal 16 is coupled out of housing 12 of device 10 throughan optical window 42. To perform a measurement, device 10 is directed ata target object 20, whose distance from the measurement device is to bedetermined. Signal 17, which is reflected or scattered on the desiredtarget object 20, forms a returning measurement beam bundle 44, acertain portion of which enters device 10 again. Returning measurementbeam 17 is coupled into the measurement device through an entry window46 in end face 48 of device 10 and directed to a receiving lens 50.Receiving lens 50 bundles the returning measurement beam bundle 44 ontoactive surface 52 of a receive device 54.

This receive device 54 can be a junction-type detector or a photodiode,for example, and a direct-mixing avalanche photodiode of a known type,for example. Active surface 52 of receive device 54 is a correspondingdetection element. Receive device 54 converts incoming light signal 17into an electrical signal, which is then forwarded via correspondingconnecting means 56 to a control and evaluation unit 58 of device 10.Control and evaluation unit 58 determines—based on returning opticalsignal 17 and, in particular, the phase displacement impressed on thereturning signal in comparison with the signal sent originally—thesought-after distance between device 10 and target object 20, anddisplays it in an optical display device 60 of the measurement device,for example.

In the case of a laser distance measurement using phase-displacementmeasurement of amplitude-modulated light, the phase displacement betweenthe light returning from target object 20 and received in detector 54and the light emitted from measurement device 10 in the direction oftarget object 20 is given by the equation: $\begin{matrix}{\varphi = {\frac{2\pi*f}{c}*2d}} & (1)\end{matrix}$

Wherein φ represents the phase displacement impressed on the lightsignal resulting from a distance d between measurement device 10 andtarget object 20, f represents the modulation frequency of theamplitude-modulated measurement signal, and c is the phase velocity(speed of light) of the measurement signal that is utilized.

The signal-to-noise ratio of the measurement signal that is useddetermines the accuracy in the determination of the distance d ofmeasurement device 10 to target object 10 in the laser distancemeasurement using phase displacement measurement.

The measurement uncertainty Δφ in a phase measurement is given by theequation: $\begin{matrix}{{\Delta\varphi} = \frac{1}{\sqrt{2*\frac{S}{N}}}} & (2)\end{matrix}$

The signal-to-noise ratio S/N, which determines the measurementuncertainty, may be determined, for example, based on an amplitudemeasurement of the modulation signal and the direct component of ambientlight that results in a corresponding noise in the measurement signal.

Since the signal-to-noise ratio can basically be measured, it is alsopossible according to the invention to influence a distance measurementsuch that a predetermined target value for the signal-to-noise ratio S/Nand, therefore, for the measurement uncertainty A(p, is achieved in thephase measurement, e.g., by adjusting the measurement time. With themethod according to the invention, the target signal-to-noise ratio tobe achieved in a measurement can be set by the user indirectly in theform of a preselected measurement time, e.g., via an operating field 62of the control and evaluation unit 58 of measurement device 10, orautomatically or semi-automatically in optimized fashion by themeasurement device itself.

Using a short distance measurement carried out before the actualmeasurement procedure, for example, an erroneous rough estimate of thesought-after distance can therefore be carried out, followed by a moreexact measurement, which is carried out, however, with a requirement onthe measurement uncertainty and, therefore, the signal-to-noise ratioS/N that is adjusted to the rough distance range.

A subset can also be selected from a series of distance measurements toadjust the measurement uncertainty, of the determined measurementdistance, for example, based on these results. Since an increasingnumber of individual measurements, e.g., with different frequencies, iscarried out to determine a distance, individual measurements of thistype can be utilized to carry out information for adjusting measurementuncertainty. This means that the measurement uncertainty can also beadjusted and optimized during the determination of a distance of themeasurement task.

As an alternative, the measurement range accessible by the measurementdevice can be expanded within a predetermined maximum measurement timeby reducing the signal-to-noise ratio requirements across the distance.Especially in the outdoors with strong sunlight, which results in araised noise level, this can result in a marked increase of themeasurement distance that is possible with measurement device 10according to the invention and, therefore, in an increase in theusability of the measurement device according to the invention. Theaccuracy of the distance measurement, which decreases as the distanceincreases, can be visualized and communicated to the user by reducingthe resolution of the display of the measured results in display 60 ofmeasurement device 10.

FIG. 2 shows an exemplary embodiment of the essential steps of themethod according to the invention using a flow chart of individualmethod steps.

At the beginning of the method, a measurement time for the upcomingdistance measurement is defined in method step S1. It is translatedinside the device into a target for the number n of sampling periods ofthe modulated measurement signal that are used by the evaluation unit toevaluate the measurement signal. The desired measurement time can becommunicated to the measurement device and/or the evaluation unit of themeasurement device manually by the user, e.g., via operating field 62,or automatically by a corresponding routine of the control andevaluation unit 58 of device 10.

After the measurement time is specified, a measurement is started, e.g.,by actuating a corresponding “Start” button in operating field 62 ofmeasurement device 10, a measurement signal 16 is emitted from thedevice in the direction of sighted target object 20, and measurementsignal 17 reflected on target object 20 is detected by the measurementdevice. For known reasons and reasons cited in publication DE 198 11 550A1, for example, it can be advantageous to repeat this measurementprocedure with measurement signals having a different frequency. Tosimplify the further description of the method according to theinvention, only the method for one frequency will be described below.

In method step S2, the amplitude-modulated measurement signal isdetected and processed further in accordance with the previouslyselected measurement time over a period of n periods. In method step S3,the amplitude of the detected measurement signal is determined from themeasurement signal arriving at receive detector 54 and, in a parallel orserial method step S4, the noise portion contained in the measurementsignal is determined.

In method step S5 according to FIG. 2, the signal obtained from theamplitude determination is converted to a ratio with the noise portiondetermined in method step S4, thereby calculating the signal-to-noiseratio S/N on which the completed measurement is based.

In a method step S6, which is parallel to the measurement procedure, adesired, theoretical accuracy target is transmitted to the measurementdevice in the form of measurement uncertainty.

This can take place via manual input by the user before the actualmeasurement, or via an automatic or semi-automatic assignment by themeasurement device itself. For example, the measurement device can alsoaccess a memory internal to the device, in which values for themeasurement uncertainty are stored. These values can be stored as afunction of distance ranges, for example, so that a smaller measurementuncertainty is used for a measurement in the range of 1 m to 3 m than inthe range of 5 m to 20 m or in the range of 20 m to 100 m, for example.Various characteristic curves can also be stored in the measurementdevice itself, the characteristic curves reflecting the differentfunctional interrelationships between the measurement uncertainty onwhich the measurement is based and the distance to be measured.

Based on the accuracy target in method step S6, i.e., based on theselected measurement uncertainty, the associated, necessarysignal-to-noise ratio that must be adhered to to attain the measurementuncertainty according to method step S6 is calculated in method step S7.

By using appropriate sensors, the measurement uncertainty to be appliedcan be adapted to the environmental parameters. For example, an adjustedmeasurement uncertainty can be selected with consideration for the levelof the background signal and the desired measurement time, i.e., asignal-to-noise ratio can be specified, up to which the measurementshould be carried out. The environmental parameters do not necessarilyhave to be purely optical environmental parameters. Using appropriatesensors, for example, any other type of radiation, e.g., cell phoneinterference, radar signals or “electro smog”, can be detected whichcould influence the signal-to-noise ratio. Using the control andevaluation unit of the device, the measurement uncertainty can then beset in a manner yet to be described.

At the same time, in method step S8, the resolution of display 60 ofmeasurement device 10 is adjusted by the central control and evaluationunit 58 of measurement device 10 according to the invention to theaccuracy target according to method step S6. For example, by reducingthe number of decimal places in the depiction of the measurementresults, the user can be informed as to which measurement accuracy ormeasurement uncertainty the completed measurement was based on.

It is also possible, for example, using corresponding operating buttonsin operating field 62 of measurement device 10, to indicate the numberof decimal places in the display, e.g, before a measurement, and therebynotify control and evaluation unit 58 directly as to which measurementuncertainty the subsequent distance measurement is to be carried out.The device can then also call up a stored characteristic curve, forexample. It is also possible to specify to the device the distance rangein which the subsequent distance measurement will be located, so that acorresponding measurement accuracy can be selected semi-automatically bythe device.

A comparison is carried out in method step S9 between the desired “S/Ntarget” signal-to-noise ratio according to method step S7 and the “S/Nactual” signal-to-noise ratio on which the actual measurement is based.If the measured actual value of the signal-to-noise ratio does notcorrespond to the targets of the actual value according to method stepS6, the measurement time required to reach the target value iscalculated and, out of this, the required number of measurement periodsn for the evaluation unit is determined. In this case, the methodbranches off back to method step S2, so that a renewed measurement withthe now-adjusted measurement time is started and/or the on-goingmeasurement is carried out or continued with the now-adjusted number ofsampling periods.

If it should arise that the measurement time required for thecorresponding distance measurement with the required measurementuncertainty is too great, or if a predetermined measurement time wereexceeded, it is provided that the measurement device automaticallyrounds the measurement uncertainty up. In this case, the method branchesback to method step S6, in which the measurement uncertainty isspecified. The decision in method step S6 can then be made by selectinganother characteristic curve of the measurement uncertainty as afunction of the distance, or by specifying a fixed value for themeasurement uncertainty. To this end, the measurement device accordingto the invention can also “scroll through” the individual characteristiccurves of the measurement uncertainty in order to find the measurementuncertainty that just allows a measurement to be carried out in thedesired measurement time.

If the measured “S/N actual” signal-to-noise ratio corresponds to thedesired “S/N target” signal-to-noise ratio, the distance between themeasured device and the target object is determined in method step S10in known fashion based on the phase displacement determined over nperiods of the modulated measurement signal. The method disclosed inpublication DE 198 11 550 A1 can be used for distance measurement, forexample.

In final method step S11, the distance between measurement device 10 andtarget object 20 determined by evaluation unit 58 is depicted in display60 of measurement device 10, whereby to visualize the measurementuncertainty on which the measurement is based, the accuracy of thedepicted distance value corresponds to the resolution of thecorresponding predetermined measurement uncertainty.

The method according to the invention may be stored as a correspondingroutine in the form of a control program, e.g., in control andevaluation unit 58 of a distance measurement device 10, so that anautomatic or semi-automatic variation of the measurement uncertainty canalso be carried out by the device itself, as a function of themeasurement parameters. To this end, the corresponding characteristiccurves can be stored in a storage medium and read out by the control andevaluation unit.

FIG. 3 shows, in a schematic manner, various examples of curves for themeasurement uncertainty δ on which a distance measurement is to bebased, as a function of a measurement distance D. Curve a represents themeasurement uncertainty that results alone based on the systematic errorof the quartz oscillator that defines the measurement frequencies of thedevice. As indicated in equation (1), fluctuations in the frequency ofthe measurement signal also result in corresponding phase displacementsin the signal that appear in errors for the distance to be determinedtherefrom and therefore contribute to measurement uncertainty. Thismeasurement uncertainty reflected in curve a is therefore a measurementuncertainty that is internal to the device and can be optimized for themeasurement device only by selecting qualitatively high-qualityelectronic components.

Curve b shows the measurement uncertainty that results when anadditional statistical error is present due to a fixed signal-to-noiseratio S/N. Curve b therefore approximately reflects the minimummeasurement uncertainty attainable with a measurement device as afunction of measurement distance D.

Curves c, d, e and f show possible characteristic curves for themeasurement uncertainty that can be stored in the device according tothe invention. The characteristic curves can also have a non-linearfunction course and are not limited to the functional dependenciesdepicted in FIG. 3. When performing a distance measurement, measurementdevice can thereby successively “scroll through” these characteristiccurves in order to not exceed a measurement time T₀ that may bepredetermined. An optimization routine in the control and evaluationunit of the measurement device can then select that characteristic curvefor a certain measurement distance that represents the optimalcompromise between measurement time and measurement accuracy, withconsideration for the measurement time required for this distancemeasurement.

FIG. 4 shows, also in a simplified, schematic representation, themeasurement times B through E—corresponding to characteristic curves bthrough e in FIG. 3—as a function of the measured distance D. It isclear to see that the distance range D₀ yet to be measured over acertain measurement time T₀ can be markedly expanded by the selection,that is, by the free specification of a measurement uncertainty to thedevice by the device itself. The measurement uncertainty which can bespecified to the device can also be located markedly above themeasurement uncertainty that is specified as being conditional upon thedevice, as shown in FIG. 3, for example.

The method according to the invention and the corresponding deviceaccording to the invention therefore make it possible to expand thedistance range usable with a measurement device for distancemeasurement, that is, that distance range across which a distancemeasurement can be carried out with the device, using simple means.

The method according to the invention and the device according to theinvention for carrying out this method are not limited to the exemplaryembodiments depicted in the description.

In particular, the method according to the invention and thecorresponding measurement device for carrying out the method are notlimited to the use of a phase measurement principle. Distancemeasurement devices that function according to the transit timeprinciple, for example, can also make use of the method according to theinvention.

Furthermore, the method according to the invention is not limited to usein optical distance measurement devices. The method according to theinvention can also be used in ultrasound devices for distancemeasurement.

1. A device for distance measurement, with at least one transmittingbranch (14) with at least one transmitter (22, 24) for emitting amodulated measuring beam (16, 26, 36) in the direction of a targetobject (20), and with at least one receive branch (18) for receiving themeasurement radiation (17, 44) returning from the target object (20),and with a control and evaluation unit (28, 58) for determining thedistance of the device to the target object (20), wherein the device hasmeans that enable a measurement of distances with predeterminedmeasurement uncertainties.
 2. The device as recited in claim 1, whereinthe means are configured such that the measurement uncertainty on whicha distance measurement is based is adjustable as a function of themeasurement distance to the target object (20) and/or of the measurementtime of the distance measurement.
 3. The device as recited in claim 1,wherein the device includes a storage medium (64) in which themeasurement uncertainty on which a distance measurement is based isstored in the form of data, in particular in the form of at least onecharacteristic curve.
 4. The device as recited in claim 3, wherein thestorage medium includes a plurality of characteristic curves for themeasurement uncertainty, in particular characteristic curves (a, b, c,d, e, f) of the measurement uncertainty as a function of the measurementdistance, which said characteristic curves can be called up for adistance measurement using operating elements (62) on the measurementdevice.
 5. The device as recited in claim 3, wherein the storage medium(62) includes a plurality of characteristic curves for the measurementuncertainty, in particular characteristic curves (a, b, c, d, e, f) ofthe measurement uncertainty as a function of the measurement distance,which said characteristic curves can be called up selectively orsuccessively for a distance measurement by the control and evaluationunit (28, 58) of the measurement device.
 6. The device as recited inclaim 1, wherein the device includes means that permit the measurementuncertainty on which a distance measurement is based to be set byproviding a default signal-to-noise ratio, in particular asignal-to-noise ratio of the measurement signal, to the control andevaluation unit (28, 58) of the device.
 7. The device as recited inclaim 6, wherein the device includes means that allow the value of thesignal-to-noise ratio to be attained in a measurement procedure to bespecified to the device before a measurement procedure.
 8. The device asrecited in claim 6, wherein the device includes means that allow atleast one measurement parameter from a group of parameters that includesat least the measurement time of the device for a distance measurementand the measurement uncertainty of the device for a distance measurementto be quantitatively preselected by the user of the device so that theother measurement parameters of this group can be adjustedsemi-automatically using the electronics of the measurement device suchthat a predeterminable value of a signal-to-noise ratio of a measurementvariable, in particular of the signal-to-noise ratio of the measurementsignal, is achieved in a measurement procedure.
 9. The device as recitedin claim 6, wherein the device includes means that allow a set ofparameters that contains at least the measurement time for a distancemeasurement using the device and the measurement uncertainty of thedevice for a distance measurement to be adjusted fully automatically forthe particular measurement distance and/or the environmental conditions,in particular for the reflectance of a target object and/or theintensity of the ambient light such that a predetermined value of asignal-to-noise ratio, in particular the signal-to-noise ratio of themeasurement signal, is achieved in a distance measurement.
 10. Thedevice as recited in claim 6, wherein the device includes means thatenable the predetermined limiting value of the signal-to-noise ratio fora distance measurement to be determined by a first measurement of thedistance—carried out before the actual distance measurement—withincreased measurement uncertainty.
 11. The device as recited in claim 6,wherein the device includes means that make it possible for thepredetermined limiting value of the signal-to-noise ratio for a distancemeasurement to be obtained based on partial results of a series ofdistance measurements.
 12. The device as recited in claim 1, wherein thedevice includes an output unit, in particular a display unit (60), thatmakes it possible to reproduce the measurement uncertainty used in adistance measurement and/or to reproduce the distance resolution of thedevice.
 13. The device as recited in claim 1, wherein the deviceincludes an output unit, in particular a display unit (60), that makesthe measured result of a distance measurement depictable with the numberof decimal places of the measured value of the distance corresponding tothe measurement uncertainty.
 14. The device as recited in claim 1,wherein the transmitter of the device includes at least one light source(22), in particular at least one laser (24).
 15. A method for distancemeasurement, with which at least one transmitting branch (14) of ameasurement device (10) emits a modulated measuring beam (16, 26, 36) inthe direction of a target object (20), the measuring beam (17, 44)reflected by the target object (20) and returning is detected in themeasurement device (10), and the distance of the measurement device (10)to the target object (20) is capable of being determined based on thereflected measurement signal, wherein the particular value of themeasurement uncertainty on which a measurement of the distance to atarget object is to be based, is capable of being specified to themeasurement device.
 16. The method for distance measurement as recitedin claim 15, wherein the value for the measurement uncertainty on whicha distance measurement is to be based is capable of being set as afunction of the measurement distance to the target object (20) and/or ofthe measurement time of the distance measurement.
 17. The method fordistance measurement as recited in claim 15, wherein the value of themeasurement uncertainty on which a distance measurement is to be basedtakes place by providing a target value for a signal-to-noise ratio, inparticular the signal-to-noise ratio of the measured signal to beattained in a measurement procedure.
 18. The method for distancemeasurement as recited in claim 17, wherein the predetermined limitingvalue of the signal-to-noise ratio for a distance measurement isdetermined via a first measurement of the distance—carried out beforethe actual distance measurement—with increased measurement uncertainty.19. The method for distance measurement as recited in claim 17, whereinthe predetermined limiting value of the signal-to-noise ratio for adistance measurement is obtained based on partial results of a series ofdistance measurements.
 20. The method for distance measurement asrecited in claim 15, wherein the value for the measurement uncertainty,on which a distance measurement is to be based, is stored in themeasurement device in the form of one or more characteristic curves andcan be called up and/or selected automatically by the device and/or bythe user of the device.
 21. The method for distance measurement asrecited in claim 19, wherein the value for the measurement uncertaintyon which a distance measurement is to be based is optimized by selectinga characteristic curve stored in the device.
 22. The method for distancemeasurement as recited in claim 20, wherein the value for themeasurement uncertainty on which a distance measurement is to be basedis optimized by specifying a maximum measurement time for the distancemeasurement by selecting a characteristic curve (a, b, c, d, e, f) forthe measurement uncertainty.
 23. The method for distance measurement asrecited in claim 15, wherein the value for the measurement uncertaintyis increased incrementally during the distance measurement until thedistance measurement is possible in a predetermined period of time.